Six months later and I finally solved it! Actually I put DCS aside for several months and just came back to it, and this problem continued to bug me, so I kept at it. Initially the different folder structure of the F-15E compared to say, the F-16C and F/A-18C, which had known solutions to the same problem, confused me, as I was trawling through several LUA files trying to find the same parameters.
But then finally today I found the very simple solution, which actually is the same fix that the F-16 and F-18 use - I hadn't realized the F-15E has the same file: you have to edit the Display_StrokeDefs.lua file in X:\DCS World OpenBeta\Mods\aircraft\F-15E\Cockpit\Displays\.
Default vaules are:
stroke_thickness = 0.6
stroke_fuziness = 0.4
Which I changed to 0.2 and 0.4, respectively, which looks good on my 7680x2160 resolution. Hope this helps somebody else too!
GregP's post in HUD font too thick at high FOVs was marked as the answer